It should be recalled that burnishing is a technique that performs surface plastic deformation by pressing a rotary or sliding tool against the surface of a part that has already been roughed out. As it moves, the tool compresses the microscopic peaks in the surfaces concerned into the adjacent hollows, thereby enabling said surfaces to be densified.
Burnishing thus serves simultaneously to smooth surfaces and to put such surfaces into compression. The resulting mechanical forces, both on the surface and down to a certain depth, enable the lifetime of materials and structures that are subjected to cyclic changes (fatigue) or to contact corrosion to be considerably increased. This technique appears to be even more effective than shot blasting for obtaining surface compression stress, and it very considerably increases fatigue life, resistance to corrosion under tension, and resistance to the effect of corrosion due to rubbing.
As a result, burnishing is a technique that is most advantageous for metal parts that are particularly at risk, as is the case for the wheels of aircraft landing gear, e.g. wheels made of aluminum or magnesium light alloy.
Burnishing should thus be applied to regions of parts that are subjected to heavy loading, and also to regions where stress concentrations are to be feared (circular grooves, spokes, and connection webs, for example). This operation is performed by applying a force by means of one or more rotary burnishing disks, said disk(s) often also being driven in a forwards direction. This force may be applied in a manner that is advantageous by using a disk connected to a moving tool carrier by means of a flexure bar.
Disk installations have already been proposed (EP-A-0 330 743) making use of a pair of parallel spring blades that are interchangeable, thereby enabling the thrust force to be varied by selecting spring blades of a stiffness that is most suitable for the part.
Modern burnishing techniques use a tool carrier that can be associated with a numerically-controlled machine tool. In particular, when the parts are circularly symmetrical, the machine tool is a lathe and it rotates the part to be burnished, and it has a tailstock (on which the tool carrier for the disk is mounted) that is movable along two orthogonal axes, one of which axes (perpendicular to the axis of revolution of the parts) enables the disk to be pressed against the surface to be worked, and the other axis (parallel to the above-mentioned axis of revolution) enables the disk to follow the profile of the part.
Persons skilled in the art are well aware that the force with which the disk is pressed must be adjusted as a function of the type of part concerned, and also as a function of the material from which said part is made.
However, this adjustment is difficult and essentially empirical, and optimum burnishing conditions are often found only after multiple inspections of parts after burnishing. Such inspections are generally of the destructive type, and this constitutes a non-negligible drawback when the parts are sophisticated in structure and relatively high in cost, as is the case, for example, with light alloy landing gear wheels, e.g. made of magnesium or of aluminum alloy.
For burnishing light alloy airplane wheels, U.S. Pat. No. 4,835,826 teaches the use of a burnishing disk support connected via an omega spring to the tailstock of a numerically-controlled machine, which tailstock is displaced under program control (as a function of the shape of the wheel and of the thickness of the regions concerned), with the pressure exerted by the disk then being given by the programmed displacement of said tailstock. GB-A-881 229 teaches the use of a burnishing disk support which is connected firstly by spring blades to a manually positioned tool carrier, and secondly to the rod of a pneumatic actuator having a diaphragm that exerts the required thrust pressure on the disk: in that document it is specified that such a floating resilient mount for the disk makes it possible to avoid variations in the thrust force that result from the wheel to be burnished not being exactly circular, and that a radius of curvature of 3 mm is suitable for burnishing airplane wheels.
None of those techniques makes it possible to avoid the above-mentioned inspections of parts after burnishing for the purpose of optimizing burnishing conditions.
In addition, the local deformation by compression that results from burnishing is plastic, and thus irreversible, and as a result exceeding the desired values for surface compression stresses gives rise to the burnished part being rejected.
As a result, it is necessary to increase the number of preliminary adjustments and inspections so as to achieve the best burnishing conditions, and to program accordingly the machine tool that is going to perform the burnishing process.